Discreet event operators for event management in a wireless sensor network

ABSTRACT

A wireless sensor network comprises a plurality of nodes that communicate over wireless communication links. At least one of the plurality of nodes receives sensor data from a sensor. The wireless sensor network is queried by specifying a set of events of interest. For each event of interest included in the set of events of interest, a producer node included in the plurality of nodes identifies when that event of interest occurs and, when that event of interest occurs, transmits event data related to that event of interest to a consumer node included in the plurality of nodes. The query is expressed in a discrete event process algebra that is used to manage processing performed to implement the query.

CROSS REFERENCE TO RELATED CASES

This application is related to the following applications filed on evendate herewith, all of which are hereby incorporated herein by reference:

U.S. patent application Ser. No. 10/974,216, entitled “EVENT-BASEDFORMALISM FOR DATA MANAGEMENT IN A WIRELESS SENSOR NETWORK.”

U.S. patent application Ser. No. 10/974,073entitled “LAYEREDARCHITECTURE FOR DATA MANAGEMENT IN A WIRELESS SENSOR NETWORK.”

U.S. patent application Ser. No. 10/974,362, entitled “PUBLISH/SUBSCRIBEMODEL IN A WIRELESS SENSOR NETWORK.”

U.S. patent application Ser. No. 10/974,198, entitled “MACHINEARCHITECTURE FOR EVENT MANAGEMENT IN A WIRELESS SENSOR NETWORK.”

TECHNICAL FIELD

The following description relates to wireless sensor networks in generaland to data management in a wireless sensor network in particular.

BACKGROUND

Systems often include some type of functionality for providing datamanagement. Data management is concerned with providing a logical viewof the data that is available in a system. Such a logical view is alsoreferred to here as the “data model” for the system. Data management isalso concerned with the underlying physical organization of the data inthe system and the transformation between the logical view of the dataand the underlying physical organization. In addition, data managementis typically concerned with a query mechanism for retrieving data fromthe system, a frame structure for the data, and the optimization ofqueries based on various parameters.

One type of system is a wireless sensor network. A wireless sensornetwork typically include several nodes that communicate with oneanother over wireless communication links (for example, over radiofrequency communication links). One or more of the nodes in the wirelesssensor network incorporate (or are otherwise coupled to) a sensor. Suchnodes are also referred to here as “wireless sensor nodes” or “sensornodes.” Each sensor is capable of generating a value that, at any givenpoint in time, is indicative of some physical attribute of interest. Inone configuration, the sensor nodes are battery powered and have limitedcomputational resources (for example, limited memory and processingcapability).

One approach to providing data management in a sensor network employstechniques used in relational database management systems (RDBMS). Insuch an approach, sensor data generated by sensor nodes in the networkare logically organized into tables. Relational algebra is used forspecifying the behavior of the logical view of the sensor data. Such anRDBMS approach, however, may not be suitable in a wireless sensornetwork that makes use of sensor nodes that have limited resources (forexample, power, memory, or processing capability).

SUMMARY

In one embodiment, a wireless sensor network comprises a plurality ofnodes that communicate over wireless communication links. At least oneof the plurality of nodes receives sensor data from a sensor. Thewireless sensor network is queried by specifying a set of events ofinterest. For each event of interest included in the set of events ofinterest, a producer node included in the plurality of nodes identifieswhen that event of interest occurs and, when that event of interestoccurs, transmits event data related to that event of interest to aconsumer node included in the plurality of nodes. The query is expressedin a discrete event process algebra that is used to manage processingperformed to implement the query.

In another embodiment, a wireless sensor network comprises a pluralityof nodes that communicate over wireless communication links. At leastone of the plurality of nodes receives sensor data from a sensor. Thewireless sensor network is queried by specifying a set of events ofinterest. For each event of interest included in the set of events ofinterest, a producer node included in the plurality of nodes identifieswhen that event of interest occurs and, when that event of interestoccurs, transmits event data related to that event of interest to aconsumer node included in the plurality of nodes. The wireless sensornetwork further comprises a data management stack that comprises aplurality of layers. The plurality of layers comprises a query formalismlayer that receives a query that identifies the set of events ofinterest and expresses the query in a discrete event process algebrathat is used to manage processing performed to implement the query.

The details of various embodiments of the claimed invention are setforth in the accompanying drawings and the description below. Otherfeatures and advantages will become apparent from the description, thedrawings, and the claims.

DRAWINGS

FIG. 1 is a block diagram of one exemplary embodiment of a wirelesssensor network.

FIG. 2 is a block diagram of one embodiment of a wireless sensor node.

FIG. 3 is a block diagram of one embodiment of a logical source entitythat generates an event of interest in a wireless sensor network.

FIG. 4 is a high-level flow diagram of one embodiment of a method ofgenerating an event of interest using the source entity of FIG. 3.

FIG. 5 illustrates one example of a subscription request in the wirelesssensor network of FIG. 1.

FIG. 6 illustrates, generally, one example of a recursive subscriptionrequest in the wireless sensor network.

FIG. 7 is block diagram of one embodiment of a data management stack forproviding data management functionality in a wireless sensor network.

FIG. 8 is a Backus-Naur Form of one example of a high-level querylanguage.

FIG. 9 is a flow diagram of one embodiment of a method of compiling asource form of a query in order to generate the binary form of thatquery.

FIG. 10 is a block diagram illustrating one exemplary instruction formatfor use with the method of FIG. 9.

FIG. 11 illustrates one example of an instruction set.

FIG. 12 illustrates one example of a query expressed in source formusing the grammar set forth above in FIG. 8.

FIG. 13 illustrates a graph generated from the query that is expressedin source form in FIG. 12.

FIG. 14 is a table having a row for each node in the graph shown in FIG.13.

FIG. 15 is a binary form of the query set forth in FIG. 12.

FIG. 16 illustrates one example of a set of recursive subscriptions thatcan result from the query of FIG. 12.

FIG. 17 is a block diagram of one embodiment of a virtual machine foruse in a wireless sensor network.

FIG. 18 is a flow chart illustrating the processing of an event programby the embodiment of the virtual machine shown in FIG. 17.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of one exemplary embodiment of a wirelesssensor network 100. The wireless sensor network 100 includes multiplewireless sensor nodes 102 that communicate with one another and/or abase station 104 using wireless communication links. The nodes of thewireless sensor network 100, in some embodiments, are distributed over alarge geographical area. In one embodiment of the wireless sensornetwork 100, wireless sensor nodes 102 are distributed over anenvironment that is to be monitored. Each wireless sensor node 102includes (or is otherwise coupled to) a sensor that is capable ofgenerating a value that, at any given point in time, is indicative ofsome physical attribute of interest. Each wireless sensor node 102receives sensor data from a respective sensor.

In one embodiment, the wireless sensor nodes 102 and the base station104 communicate with one another using radio frequency (RF)communication links. In other embodiments, other wireless communicationlinks (for example, infrared wireless communication links) are usedinstead of or in addition to RF wireless communication links. In oneembodiment, the wireless sensor network 100 is implemented as an ad-hoc,peer-to-peer network. In such an embodiment, the nodes of the wirelesssensor network 100 communicate with each other wirelessly using amulti-hop protocol. Such a multi-hop protocol provides a mechanism for apacket (or other unit of data) to be transmitted by a source node to adestination node outside of the wireless transmission range of thesource node by transmitting the packet to an intermediate node withinthe source node's wireless transmission range. The intermediate nodethen forwards the packet onto the destination node (if the destinationnode is within the intermediate node's wireless transmission range) oronto another intermediate node within the first intermediate node'swireless transmission range. This forwarding process is repeated untilthe packet reaches the destination node. In another embodiment, thewireless sensor network 100 is implemented using a different wirelessnetworking approach (for example, using an infrastructure wirelessnetwork in which wireless communications are routed through an accesspoint).

The base station 104 provides a static point from which queries can beinjected into the wireless sensor network 100 and from which data thatis retrieved by such queries can be received. In one embodiment, a usercommunicates a query to the base station 104. The base station 104receives the query and injects the query into the wireless sensornetwork 100. The query propagates to appropriate sensor nodes 102, whichcommunicate data back to the base station 104 (via one or moreintermediate nodes) as specified in the query. In one implementation,the base station 104 also acts as a gateway to another network or devicenot otherwise included in the wireless sensor network 100 from whichqueries are received and/or to which data retrieved from the wirelesssensor network 100 is communicated.

The wireless sensor network 100 can also include other types of nodes.For example, as shown in FIG. 1, a personal digital assistant (PDA) 106is included in the network 100. The PDA 106 includes a wirelesstransceiver that enables the PDA 106 to communicate with other nodes inthe wireless sensor network 100 over one or more wireless communicationlinks. In one usage scenario, a user uses the PDA 106 to input a queryfor data from the wireless sensor network 100. The PDA 106 communicatesthe query to the base station 104 (via one or more intermediate nodes,if necessary). The base station 104 receives the query and injects thequery into the wireless sensor network 100 and communicates back to thePDA 106 any data received from the wireless sensor network 100 inresponse to the query.

In the exemplary embodiment shown in FIG. 1, at least a portion of thenodes in the network 100 are logically arranged into regions 108. Aregion 108, in such an embodiment, defines a geographic area. Eachregion 108 is considered to include those nodes that are physicallylocated within the geographic area of that region 108. For example, asshown in FIG. 1, the wireless sensor nodes 102 and the PDA 106 arearranged into four regions 108 (specifically, identified in FIG. 1 asregion A, region B, region C, and region D). In such an embodiment, auser or application that retrieves data from the network 100 using theregions 108, if appropriate for the needs of that user or application.In other embodiments, such logical regions 108 are not used.

FIG. 2 is a block diagram of one embodiment of a wireless sensor node102. The wireless sensor node 102 shown in FIG. 2 is suitable for use inthe embodiment of a wireless sensor network 100 shown in FIG. 1. Theembodiment of a wireless sensor node 102 shown in FIG. 2 comprises asensor interface 202 that couples a sensor 204 to the wireless sensornode 102. In the particular embodiment shown in FIG. 2, the sensor 204is integrated into the wireless sensor node 102 (for example, byenclosing the sensor 204 within a housing that encloses the sensor 204along with the other components of the wireless sensor node 102). Inanother embodiment, the sensor 204 is not integrated into the wirelesssensor node 102 but is otherwise communicatively coupled to the othercomponents of the wireless sensor node 102 via the sensor interface 202.

The sensor 204 is capable of generating a value that, at any given pointin time, is indicative of some physical attribute of interest. Examplesof sensors 204 include devices that generate a value indicative oftemperature, light, magnetic field, air flow, acceleration, vibration,sound, or power. The sensor interface 202 comprises appropriateinterface hardware or software for communicatively coupling the sensor204 to the other components of the wireless sensor node 102. Forexample, in one embodiment, the software interface 202 includes, forexample, an analog-to-digital converter and/or a software driver for thesensor 204.

The wireless sensor node 102 shown in FIG. 2 further comprises aprogrammable processor 206. The programmable processor 206 is programmedwith appropriate program instructions to perform at least a portion ofthe processing described here as being performed by the wireless sensornode 102. The wireless sensor node 102 shown in FIG. 2 includes memory208 in which such program instructions and any data structures used bythe program instruction are stored. The memory 208 includes anyappropriate type of memory now known or later developed includingwithout limitation, read-only memory (ROM), random access memory (RAM),and a set of registers included within the processor 206.

The wireless sensor node 102 shown in FIG. 2 also comprises a wirelesstransceiver 216 that transmits and receives data over one or morewireless communication links. In one embodiment, the wirelesstransceiver 216 comprises a RF transceiver that sends and receives dataover one or more RF communication links. In other embodiments, thewireless transceiver 216 comprises other types of wireless transceiversfor sending and receiving data over other types of wirelesscommunication links (for example, an infrared transceiver for sendingand receiving data over infrared communication links) instead of or inaddition to an RF transceiver.

The wireless sensor node 102 also comprises a power source 218. In theembodiment shown in FIG. 2, the power source 218 includes a battery 220.In other embodiments, the power source 218 comprises, in addition to orinstead of a battery 220, an interface for coupling the wireless sensornode 102 to an external power source such as a source of alternatingcurrent (AC) power. The wireless sensor node 102 also comprises one ormore hardware timers 222 that are used generating interrupts based ontiming-related events.

In one implementation of the embodiment shown in FIG. 2, the wirelesssensor node 102 is implemented using a CHIPCON CC1010 integrated circuitthat includes an 8-bit micro-controller, 32 kilobytes of flash memory,and 2 kilobytes of RAM.

In the embodiment shown in FIGS. 1 and 2, an event-based data managementmodel is used to implement data management functionality in the wirelesssensor network 100. Each of the nodes in the wireless sensor network 100includes a data management interface 110 that implements at least aportion of such data management functionality. The data managementinterface 110, in one implementation, comprises software that executeson a programmable processor included in each node.

In such an embodiment, the wireless sensor network 100 is logicallyviewed as a set of discrete events and a set of logical entities that“generate” the discrete events. The wireless sensor network 100 isqueried, in such an embodiment, by specifying a set of events ofinterest. With such an event-based data management model, a discreteevent operator algebra can be used as a formalism to specify thebehavior of such a logical system and to verify the correctness andcompleteness of the specification.

Each event of interest is logically viewed as having a logical entitythat is the source of that event. This source entity is also referred tohere as the “producer” of that event. Also, each event of interest islogically viewed as having one or more logical entities that are sinksof that event (and/or data related to that event). Each of these sinkentities is also referred to here as a “consumer” of that event orevent-related data. The data management model used in such anembodiment, in other words, makes use of a “producer/consumer model.”For each logical entity, there is a corresponding node in the network100 that physically implements the processing for that logical entity.The underlying node that implements a given source entity is alsoreferred to here as a “data source” and the underlying node thatimplements a given sink entity is also referred to here as a “datasink.” For example, where an event of interest is a function of sensordata provided by a particular sensor 204, the source entity for thatevent is implemented on a wireless sensor node 102 that is coupled tothat sensor 204 (that is, on the wireless sensor node 102 that is thedata source for the desired sensor data). It may be the case, that aparticular node in the wireless sensor network 100 implements both thesource entity and the sink entity for a given event.

FIG. 3 is a block diagram of one embodiment of a logical source entity300 that generates an event of interest 302 in a wireless sensornetwork. The source entity 300 makes use of an event filter 304 toidentify an occurrence of the event of interest 302 for which the entity300 is the sink entity. The event filter 302 comprises a condition 306and, when the condition 306 is true, the event of interest 302 isconsidered to have occurred. An event filter's condition 306 can bespecified as a function of continuous data 308 (for example, sensor datagenerated by a sensor) and/or other events 310 generated by otherentities (for example, an event for which the source entity 300 is alsoa sink entity).

FIG. 4 is a high-level flow diagram of one embodiment of a method 400 ofgenerating an event of interest using the source entity of FIG. 3.Method 400 is used to generate each of a set of events of interestspecified in a query that is received at the wireless sensor network100. A user or application that desires to retrieve information from thewireless sensor network 100 formulates the query and injects the queryinto the network 100. The query specifies a set of events of interestabout which the user or application wishes to receive information. Inone embodiment, the query is converted into a form suitable forcommunication to the nodes in the wireless sensor network 100. In oneimplementation (for example, as described below in connection with FIGS.9-15), the query is converted into a binary form of the query that isexecuted by one or more nodes. When a node receives the queryinformation, the node determines whether that node is able to serve as asource entity for any of the set of events of interest specified in thequery. If that node is able to serve as a source entity for an event ofinterest specified in the query, that node performs the processing ofmethod 400 for that event.

For a particular event of interest specified in a query, an event filteris created at the source entity for that event (block 402). The queryincludes information that is used by the source entity to create theevent filter for that event. For example, in one embodiment, the queryspecifies a condition, for each event of interest set forth in thequery, that is used to identify each occurrence of that event. Oncecreated, the event filter is used to identify when the particular eventof interest has occurred. When the event filter determines that theparticular event of interest has occurred (block 404), the source nodecommunicates data related to that event to the sink entity specified forthat event (block 406). That is, the physical node on which the sourceentity is implemented communicates the event-related data to thephysical node on which the sink entity is implemented over one or morewireless communication links provided in the wireless sensor network100.

The event-based data management model, in one such embodiment, makes useof a combined producer/consumer and publish/subscribe model. In such amodel, from a logical point of view, a sink entity that wishes toreceive data related to a particular event informs the wireless sensornetwork 100 of that entity's interest in that event. The sink entity'sinterest in that event is then communicated to an entity that is able toserve as a source entity for that event. The sink entity indicates thatit is interested in a particular event of interest by “subscribing” tothat event. A subscription is formulated and is communicated to a sourceentity for the event of interest. The subscription identifies the eventof interest (for example, by specifying a condition for use in an eventfilter that identifies that event) and the sink entity to which datarelated to the event should be sent when the event occurs. The sourceentity receives the subscription and creates an event filter for thatevent. The source entity “publishes” the event of interest when theevent occurs. That is, when the event of interest specified in thesubscription occurs, the source entity sends data related to that eventto the specified sink entity. In this way, the nodes in the wirelesssensor network 100 only monitor (and process and communicate data about)those events that are of interest to some entity in the network 100(that is, those events to which a sink entity has subscribed).

In such an embodiment, each subscription is installed at the physicalnode that implements the logical source entity for the event specifiedby that subscription. Each subscription can be installed, deferred,paused, resumed, dropped or updated (for example, by a user orapplication) at anytime during the lifetime of the query. Eachsubscription has a specified activation time when the source entity isto begin checking for and publishing occurrences of the event ofinterest. Each subscription, in such an embodiment, also has as aspecified lifetime after which the subscription ceases to exist (thatis, the source entity no longer checks for and publishes the event ofinterest). Also, in such an embodiment, each subscription can specifyhow often the event filter should be evaluated in order to check foroccurrences of the event of interest (also referred to here as the“event rate”).

FIG. 5 illustrates one example of a subscription request in the wirelesssensor network 100. In this example, a sink entity wishes to receiveinformation about an event that is associated with a particular sensor.The sink entity is implemented on the base station node 104 and thesource entity is implemented on the wireless sensor node 102 thatincludes the sensor that the sink entity is interested in. The datamanagement interface 110 on the base station 104 communicates thesubscription request to the wireless sensor node 102 on which the sourceentity is implemented (also referred to here as the “source wirelesssensor node 102). In communicating the request to the wireless sensornode 102, the data management interface 110 of the base station 104interacts with appropriate underlying layers of the networking stack toroute the subscription request to the wireless sensor node 102.

When the source wireless sensor node 102 receives the subscriptionrequest, the source wireless sensor node 102 installs the event filterassociated with that subscription request using the event-filterinformation specified in the subscription request. After thesubscription has been activated and during the lifetime of thesubscription, when the source wireless sensor node 102 determines thatthe event of interest has occurred, the source wireless sensor node 102publishes the event to the sink entity implemented on the base station104. In publishing the event, the data management interface 110 on thesource wireless sensor node 102 interacts with appropriate underlyinglayers of the networking stack to route data related to the event thatoccurred to the base station 104.

In such an embodiment, a subscription can be formulated by a user of thewireless sensor network 100 as a query. The user formulates the query ina formal and verifiable query language. The query language isimplemented as a high-level, human-readable language, which provides aninterface to access data that resides in the wireless sensor network 100and provides an interface to specify the intent of the user for thedata. The user, in such an implementation, supplies the query, specifiedin the query language, to a node in the wireless sensor network 100 (forexample, the base station 104 or the PDA 106). The node that receivessuch a query parses the query, checks the query for any syntactic orsemantic errors and converts the query into a set of subscriptions. Theconversion of the human-readable query (also referred to here as the“source” form of the query) into a set of subscriptions is also referredto here as “compiling” the query. In one embodiment, the set ofsubscriptions is expressed in a binary form that is designed forconvenient execution by the nodes in the wireless sensor network 100. Inone implementation of such an embodiment, the data management interface110 of the node that receives the query parses, checks, and compiles thereceived query and injects the query into the wireless sensor network100.

In such an embodiment, subscriptions can also be formulated by anapplication that resides in or interacts with the wireless sensornetwork 100. For example, in one implementation, the applicationformulates the subscriptions in source form using the high-level querylanguage. In another implementation, the application formulates thesubscriptions directly in binary form (thereby avoiding the need tocompile the query).

In such an embodiment, an optimization process that operates on thesource form of the query and/or an optimization process that operates onthe binary form of the query can be performed. Examples of variousoptimizations that can be performed in such an implementation aredescribed below.

In such an embodiment, recursive subscriptions are supported. Eachsubscription comprises a condition that identifies when the event ofinterest has occurred. The event of interest associated with thesubscription can be a “simple” event that is not expressed in terms ofany other events in the wireless sensor network 100 (for example, wherethe event is a function of continuous data from a sensor or a timer).That is, a simple event comprises a condition that is not a function ofany other event in the wireless sensor network 100. Alternatively, theevent of interest associated with a particular subscription can be a“complex” or “parent” event that is expressed in terms of one or moreother events (also referred to here as “child events”) in the wirelesssensor network 100. That is, such a parent event comprises a “parent”condition that is a function of one or more child events. Likewise, eachchild event can itself be a simple event or a parent event that isexpressed in terms of one or more child events.

The parent condition for such a parent event can combine the conditionfor a child event (referred to here as a “child condition”) with childconditions for zero, one, or more other child events and/or with zero,one, or more time-based conditions that specify when an event should bepublished, specify a duration for the subscription, specify anevent-rate for the subscription, and/or specify when the subscriptionshould be activated. These child conditions and/or time-based conditionscan be combined together, for example, using logical operators (such asAND, OR, or XOR). In one implementation of such an embodiment, thetime-based conditions are also implemented as events.

FIG. 6 illustrates, generally, one example of a recursive subscriptionrequest in the wireless sensor network 100. In this example, a recursivesubscription indicates that an entity (referred to here as the “parentsink entity”) wishes to receive information about an event that isassociated with a first sensor when data from a second sensor meets acertain condition. In this example, the recursive subscription comprisesa parent event that has a condition that indicates that the value of thefirst sensor is to be published when the value of the second sensor isgreater than 20. In this example, the parent sink entity is implementedon the base station 104 and the source entity (also referred to here asthe “parent source entity”) is implemented on the wireless sensor node102 that includes the first sensor. This wireless sensor node 102 isalso referred to here as the “first” wireless sensor node 102. The datamanagement interface 110 on the base station 104 communicates theoriginal subscription request to the first wireless sensor node 102,which install an event filter based on the condition specified in theoriginal subscription request.

The parent source entity also “splices” the original subscriptionrequest in order to generate a second subscription, based on theoriginal subscription received from the parent sink entity. The secondsubscription indicates that the parent source entity wishes to beinformed as to when the second sensor is greater than 20. In otherwords, this second subscription identifies a child event that is ofinterest to the parent source entity.

For this second subscription, the parent source entity is the sinkentity. The source entity (also referred to here as the “child sourceentity”) for this child event is the wireless sensor node 102 thatincludes the second sensor (also referred to here as the “second”wireless sensor node 102). The data management interface 110 of thefirst wireless sensor node 102 communicates the second subscriptionrequest to the second wireless sensor node 102, which installs an eventfilter based on the condition specified in the second subscriptionrequest.

When the second wireless sensor node 102 determines that the child eventhas occurred (that is, the value of the second sensor is greater than20), the second wireless sensor node 102 publishes the child event tothe child sink entity, which is implemented on the first wireless sensornode 102. In this example, the child sink entity is also the parentsource entity. When the parent source entity on the first wirelesssensor node 102 learns that the value of the second sensor node isgreater than 20, the first wireless sensor node 102 publishes the parentevent to the parent sink entity, which is implemented on the basestation 104. That is, the first wireless sensor node 102 provides thevalue of the first sensor to the parent sink entity implemented on thebase station 104.

FIG. 7 is block diagram of one embodiment of a data management stack 700for providing data management functionality in a wireless sensornetwork. The embodiment of the data management stack 700 shown in FIG. 7is described here as being implemented using the wireless sensor network100 and the wireless sensor node 102 of FIGS. 1 and 2, respectively.Other embodiments are implemented in other ways. One or more of thevarious layers described here are implemented on each node in thewireless sensor network 100.

In the embodiment shown in FIG. 7, the data management stack 700comprises six layers. The data management stack 700 includes a queryformalism layer 702 that provides the formal framework and language usedfor querying data from the wireless sensor network 100. For example, thequery formalism layer 702 provides the functionality for compiling thesource form of a query into a binary form and for injecting the queryinto the wireless sensor network 100. In such an embodiment, the queryformalism layer 702 implements one or more of the following features:

-   -   the ability to subscribe for an event.    -   the ability to publish events to all subscribers.    -   the ability to uniquely identify each subscription that is made.    -   the ability to uniquely identify each event and the subscription        that generates that event.    -   the ability to install a subscription, defer installation of a        subscription, pause a subscription, resume a subscription, drop        a subscription, update or modify a subscription, and activate a        subscription.    -   the ability to specify a lifetime for a subscription.    -   the ability to specify an activation time for a subscription.    -   the ability to specify aggregation operations for a        subscription, including a singe-source aggregation operation or        a multiple-source aggregation.    -   the ability to specify a subscription that executes only once        (that is, only one event is published for that subscription).    -   the ability to specify a priority for a subscription.    -   the ability to specify a subscription to one or more simple data        sources and/or one or more complex data sources.    -   the ability to specify a condition used in an event filter for a        subscription.    -   the ability to specify one or more parameter for a subscription.    -   the ability to specify one or more parameter for a publication.    -   the ability to specify during initialization a classification of        a data source as a producer or a consumer of a particular item        of data.    -   the ability to specify constraints on each subscription and        corresponding publications for that subscription, such as a        minimum power at a node to install that subscription at that        node, a minimum power to generate lower power publication, and a        minimum amount of time that a value should be stable before        publishing any event that is based on that value.

A Backus-Naur Form (BNF) of one example of a high-level query languagethat implements these features is shown in FIG. 8.

A description of various statements and clauses shown in FIG. 8 is givenbelow.

-   -   SUBSCRIBE EVENT Statement: This statement is used to specify a        subscription request. A subscription request is set by an entity        when an event of interest is to be monitored.    -   OF Clause: The OF clause is a part of a SUBSCRIBE EVENT        statement that is used to associate a parameter with a region        108 of the wireless sensor network 100.    -   ACTIVATE Clause: The ACTIVATE clause is a part of a SUBSCRIBE        EVENT statement that enables specification of the time when the        subscription should be activated by the source entity. If the        ACTIVATE clause is not specified the default value of IMMEDIATE        (explained below) is used for activation of the subscription.        The time parameter for this clause is specified in HH:MM:SS        format.    -   IMMEDIATE Clause: The IMMEDIATE clause is an optional clause of        a SUBSCRIBE EVENT statement. When used, this clause specifies        that the subscription should be activated immediately (that is,        without any delays).    -   WHEN Clause: The WHEN clause is an optional clause of a        SUBSCRIBE EVENT statement. This clause is used to specify a        condition for an event filter that is evaluated to identify when        the event of interest occurs. The event of interest is published        only when the condition is evaluated and found to be true.    -   LIFETIME Clause: The LIFETIME clause is an optional clause of a        SUBSCRIBE EVENT statement. When specified, this clause defines a        duration for which the SUBSCRIBE EVENT statement is to be kept        alive at the node where the subscription is installed. After the        duration specified in this clause has elapsed, this subscription        is “removed” from that node. The time parameter for this clause        is specified in HH:MM:SS format.    -   INFINITE Clause: The INFINITE clause is an optional clause of a        SUBSCRIBE EVENT statement. When used, it must be used along with        a LIFETIME clause in the SUBSCRIBE EVENT statement. This clause        defines the lifetime of that subscription as infinite, which        means that the subscription will be active while the node at        which the subscription is installed remains alive.    -   ONCE Clause: The ONCE clause is an optional clause of a        SUBSCRIBE EVENT statement. When specified, this clause should be        specified along with a LIFETIME clause. When this clause is        specified with the LIFETIME clause, the subscription is        evaluated only once and then removed from the node at which the        subscription is installed. This is the default value used if a        LIFETIME clause is not specified in the SUBSCRIBE EVENT        statement.    -   PUBLISH EVENT Statement: The PUBLISH EVENT statement is used to        publish an event of interest that has been subscribed to in a        corresponding SUBSCRIBE EVENT statement. A PUBLISH EVENT        statement is generated by a source entity and communicated to        one or more sink entities specified in a corresponding SUBSCRIBE        EVENT statement when the conditions specified in the SUBSCRIBE        EVENT statement are evaluated and are true.    -   VALUES Clause: The VALUES clause is used in a PUBLISH EVENT        statement to communicate the current values of one or more        parameters. This clause associates a current value for a        parameter with a parameter identifier for that parameter.    -   INSTALL EVENT Statement: The INSTALL EVENT statement is used to        install an event filter at a particular node on which a source        entity is implemented for a particular subscription. The event        filter is, for example, installed into the program memory of        that node and prepared for evaluation by that node. The        subscription to be installed is specified by the event-id        parameter of the INSTALL EVENT statement.    -   UNINSTALL EVENT Statement: The UNINSTALL EVENT statement is used        to remove a subscription from the wireless sensor network 100.        This statement causes the event specified by an event identifier        to no longer be checked for by the node on which the        corresponding subscription has been installed. The subscription        is cached for a limited period of time after being uninstalled        for use in optimization processing.    -   SUSPEND EVENT Statement: The SUSPEND EVENT statement is used to        suspend a subscription. The subscription is specified using an        event identifier and the state of the subscription is set as        “suspended.”    -   DROP EVENT Statement: The DROP EVENT statement is used to remove        a subscription from the wireless sensor network 100. This        statement causes the event specified by an event identifier to        no longer be checked for by the node on which the corresponding        subscription has been installed. Unlike with the UNINSTALL EVENT        statement, the subscription is not cached for use in        optimization processing.    -   MODIFY EVENT Statement: The MODIFY EVENT statement is used to        modify an existing subscription at runtime.    -   SET LIFETIME Clause: The SET LIFETIME clause is a part of a        MODIFY EVENT statement to modify the lifetime of an existing        subscription.    -   ACTIVATE EVENT Statement: The ACTIVATE EVENT statement is used        activate a subscription that has previously been suspended via        the SUSPEND EVENT statement. The state of the specified        subscription is changed from “suspended” to “active.”    -   DEFER EVENT Statement: The DEFER EVENT Statement is used to        defer the evaluation of the specified subscription for the        specified duration.    -   MIN|MAX|AVERAGE|SUM|MEDIAN Clauses: These clauses are used to        specify that a particular application-specific aggregation        operation should be performed as a part of a subscription. The        MIN clause indicates that a minimum value of a group of events        should be computed, the MAX clause indicates that a maximum        value of a group of events should be computed, the AVERAGE        clause indicates that an average value of a group of events        should be computed, the SUM clause indicates that a sum of a        group of events should be computed, and the MEDIAN clause        indicates that a median value of a group of events should be        computed.

The data management stack 700, in the embodiment shown in FIG. 7, alsocomprises a discrete event view (DEV) layer 704. The DEV layer 704, insuch an embodiment, is scalable so that the functions performed by theDEV layer 704 can be implemented in high-end systems and low-endsystems. For example, the source and sink functionality described belowis required in both high-end systems and low-end systems, but is scaledappropriately. In such an embodiment, the DEV layer 704 also decides onthe actions to be performed for any constraints that are specified in aquery. It may be the case, however, that such constraint decisionscannot, during operation, be evaluated and made in the DEV layer 704 dueto limitations of resources and information.

The DEV layer 704 performs a semantic check of each subscription bychecking the event-filter condition specified in each subscription tocheck that valid source and sink entities have been specified. The DEVlayer 704, as a part of such processing, checks the validity of anytime-based conditions (for example, conditions specifying when thecorresponding event should be published, the lifetime of thesubscription, an event rate for the subscription, and/or when thesubscription should be activated). The DEV layer 704, as a part of suchprocessing, checks the validity of any parameters used in theevent-filter condition. In an alternative embodiment, such semanticchecking is not performed by the DEV layer 704.

In the embodiment shown in FIG. 7, the DEV layer 704 identifies thelogical source entity and one or more logical sink entities for eachsubscription and for each corresponding publication. The DEV layer 704determines and keeps track of which physical node in the wireless sensornode 100 implements each such logical entity and buffers data for theselogical entities so as to provide an interface for users andapplications to interact with the logical entities. The DEV layer 704,in such an embodiment, also handle recursive subscriptions and thecorresponding publications. For example, the DEV layer 704 maps eachpublication generated by a source entity to the correspondingsubscription so that the “chain” specified in a recursive subscriptioncan be maintained.

In the embodiment shown in FIG. 7, the DEV layer 704 maintainsinformation about the subscriptions that exist in the wireless sensornetwork 100 at any given point in time. Such information is used by theDEV layer 704 to optimize, at least partially, new queries and/orexisting queries. In one embodiment, the optimization processingperformed by the DEV layer 704 includes optimizing recursivesubscriptions that are received by the wireless sensor network 100. Suchoptimization processing is also referred to here as“recursive-subscription optimization.” The DEV layer 704, when arecursive subscription is received, recursively converts the original,recursive subscription into a set of smaller and more efficientsubscriptions. The DEV layer 704 also maintains the proper linkage ofthe set of subscriptions to main the logical relationship specified inthe original, recursive subscription.

In one embodiment, the optimization processing performed by the DEVlayer 704 includes processing that is based on, that alters, or thatotherwise takes into account the organization of the logical entities inthe wireless sensor network 100. Such optimization processing is alsoreferred to here as “data-source organization optimization” or“data-source optimization.” As noted above, logically, the wirelesssensor network 100 can be viewed as a set of entities, each of which isable to send and/or receive data related discrete events of interest. Inthe embodiment shown in FIG. 1, the entities are organizedgeographically into regions 108. A region 108 is an abstract entity thathas an associated geographical area defined by a user of the network100. Each region 108 is considered to include all the logical entitiesthat are implemented on a physical node that is physically locatedwithin the geographical area associated with that region 108. The DEVlayer 704 can optimize a query using heuristics that are based on theorganization of the logical entities involved in the query. For example,such heuristics can be based on the physical distance between nodes usedto implement logical entities involved in a given query.

In one embodiment, the optimization processing performed by the DEVlayer 704 includes processing that is based on or that optimizes the useof available resources in the wireless sensor network 100 (for example,based on the availability of memory, power, and network bandwidth). Suchoptimization processing is also referred to here as “resource-influencedoptimization.”

In one embodiment, optimization processing performed by the DEV layer704 includes processing that is based on or that alters the status ofone or more subscriptions. Such optimization processing is also referredto here as “subscription-status influenced optimization” or“subscription-status optimization.” As noted above, at any given pointin time, many subscriptions typically exist in the wireless sensornetwork 100. Each subscription that exists in the wireless sensornetwork 100 has a current state, such as “active,” “suspended,” or“uninstalled.” Optimization processing performed by the DEV layer 704includes, for example, changing the state of a subscription in order tooptimize some attribute of the network 100 (for example, networkbandwidth used by the node on which a subscription is implemented). Inone implementation of such an embodiment, a finite state machinerepresentation of the status of each subscription is used in the DEVlayer 704, among other things, to perform such subscription-statusoptimization. Examples of heuristics that can be used in such anembodiment include the following:

-   -   if a new subscription is received by the DEV layer 704 that        specifies the same event that is specified in an existing        subscription but with an extended lifetime, the DEV layer 704        causes the node on which the existing subscription is installed        to modify the existing subscription by extending the lifetime of        the existing subscription and to publish events to both the sink        entity specified in the existing subscription and the sink        entity specified in the new subscription.    -   if a new subscription is received by the DEV layer 704 that is        the same as an existing subscription but with a different event        rate, the DEV layer 704, if possible, causes the node on which        the existing subscription is installed to modify the existing        subscription by adjusting the event rate of the existing        subscription to satisfy both the existing subscription and the        new subscription and to publish events to both the sink entity        specified in the existing subscription and the sink entity        specified in the new subscription.    -   if a new subscription is received by the DEV layer 704 that        specifies parameters similar to parameters specified in an        existing subscription, the DEV layer 704 causes the node on        which the existing subscription is installed to copy the        existing subscription information and to modify the copy in        order to install the new subscription on that node.    -   if a new subscription is received by the DEV layer 704 that is        exactly the same as an existing subscription except for        specifying a different sink entity, the DEV layer 704 causes the        node on which the existing subscription is installed to publish        events to both the sink entity specified in the existing        subscription and the sink entity specified in the new        subscription.    -   if a new subscription is received by the DEV layer 704 that        specifies a first event generated by a first source entity but a        second entity in the network 100 is also able generate a second        event that is similar to the first event, the DEV layer 704 is        able to install the subscription on the second source entity or        modify an existing subscription that is already exists on the        second source entity to publish events to the sink entity        specified in the new subscription.    -   when the lifetime of a subscription has elapsed, the node on        which that subscription is installed can be instructed to save        the subscription (for example, by changing the status of the        subscription from “installed” to “uninstalled”) for a        predetermined period of time after uninstalling the        subscription, thereby making the subscription available to the        DEV layer 704 (for example, for one or more of the optimizations        described above) for an additional period of time.

In one embodiment, optimization processing performed by the DEV layer704 includes processing that is based on, that alters, or that otherwisetakes into account a multi-source aggregation operation specified in asubscription. Such optimization processing is also referred to here as“multi-source aggregation optimization” or “multi-source optimization.”In a multi-source aggregation operation, more than one source entity isspecified for a subscription and any corresponding publications. Anexample of such a multi-source aggregation operation is a subscriptionin which one event generated by a first source entity influences anaggregation event that is generated by a second source entity. Examplesof heuristics that can be used in such an embodiment include performinglocalized data-source optimizations before performing multi-sourceoptimization, performing subscription-status optimization,resource-influenced optimization, or other kinds of optimizationprocessing before performing multi-source optimization, and/orperforming event-sequencing optimization so that events from disparatesources are ordered optimally.

In one embodiment, optimization processing performed by the DEV layer704 includes processing that is based on, that alters, or that otherwisetakes into account a single-source aggregation operation specified in asubscription. Such optimization processing is also referred to here as“single-source aggregation optimization” or “single-sourceoptimization.” In a single-source aggregation operation, a single sourceentity is specified for a subscription and any correspondingpublications. In such single-source aggregation optimization,information related to the single source entity is used in performingoptimization processing. Examples of heuristics that can be used in suchoptimization processing include implementing the source entity for suchan aggregation operation on a physical node that has higher resourceavailability and/or that results in the lowest (or lower) amount ofpower being expended to communicate with that physical node.

In one embodiment, optimization processing performed by the DEV layer704 includes processing that is based on, that alters, or that otherwisetakes into account the ordering of subscriptions and/or publications(for example, within a given subscription, region, and/or the network100 as a whole). Such optimization processing is also referred to hereas “event sequencing optimization.” In such optimization processing,dependencies between the various events and parameters are used infinding an optimal (or improved) sequencing of subscriptions and/orpublications. In one implementation, a Petri net-based model ismaintained in the DEV layer 704 for use in such optimization processing.

In one embodiment, optimization processing performed by the DEV layer704 includes processing that is based on, that alters, or that otherwisetakes into account a “when” clause specified in a subscription. The whenclause for a given subscription is used to specify a condition that isused in the event filter for that subscription. Such optimizationprocessing is also referred to here as “when-clause optimization.” Thewhen-clause optimization processing breaks down the when clause intounit/atomic conditions that are optimized using the optimizationprocessing described above. In one implementation of such an embodiment,techniques similar to those used in optimizing “where” clauses instructured query language (SQL) queries are used. For example, in onesuch implementation, a subscription/publication parse tree is assembledfrom the query and evaluated to find the cost of eachcondition-expression given in the when clause and appropriateoptimizations are performed.

In one embodiment, optimization processing performed by the DEV layer704 includes processing that is based on, that alters, or that otherwisetakes into account one or more constraints specified in a subscription.Constraints are specified for a given subscription in order to specifythe boundaries of the subscription. The constraints can be used toeliminate some of the methods or approaches used to manage thesubscription and any corresponding publications.

In one embodiment, the DEV layer 704 uses one or more finite statemachine models to implement at least a portion of the functionalityprovided by the DEV layer 704. In such an embodiment, a finite statemachine-based model is used to model those aspects of the network 100that involve specific states and state transitions. Examples of where afinite state machine-based model is used include:

-   -   tracking the status of subscriptions; each subscription        transitions through one or more of the various states mentioned        above (for example, “installed,” “active,” “suspended,”        “resumed,” and “waiting”).    -   tracking data source dependencies; global dependencies between        those entities in the network 100 that wait on other entities in        the network 100 for data or events are used to identify those        entities at which additional load can be scheduled (for example,        because those entities are in a wait state).    -   tracking the current status of entities in the network; a given        entity transitions through various states such as        “initialization,” “fault,” and diagnostics.” The states are        required during subscription scheduling optimization. For        example, when an entity is in a “diagnostic” state, operations        should not be scheduled on that entity.    -   tracking the current health status of an entity; the health of        an entity can be modeled using states and transitions between        the states. For example, the power and memory available at a        given entity can be modeled using various states and transitions        between those states can be used to represent the health of the        entity.

In one embodiment, the DEV layer 704 uses one or more Petri net modelsto implement at least a portion of the functionality provided by DEVlayer 704. In such an embodiment, a Petri net model is used in eventscheduling. The events are scheduled based on dependencies that existbetween the events. The tokens of the Petri net are used to schedule andsequence the various subscriptions and publications that exist in thewireless sensor network 100.

In the embodiment shown in FIG. 7, the data management stack 700 furthercomprises a logical layer 706. The logical layer 706 implements anabstract view of the data in the wireless sensor network 100 using theevent-based data model described above in connection with FIGS. 3through 6 in which the wireless sensor network 100 is logically viewedas a set of logical entities that generate discrete events. Such anevent-based data model makes use of a producer/consumer model in which agiven entity can be classified as a consumer of certain data and aproducer of other data. The use of the producer/consumer model, amongother things, supports sensing, actuation, and control in sensor network100. The event-based data model also makes use of a publish/subscribemodel so that the nodes in the wireless sensor network 100 only monitor(and process and communicate data about) those events that are ofinterest to some entity in the network 100.

The data management stack 700, in the embodiment shown in FIG. 7, alsocomprises an extended logical layer 708 in which applicationscenario-specific extensions to the logical layer 706 are made. In oneimplementation, the extended logical layer 708 includes extensions thatmaintain information related to event dependencies, data dependencies(for example, dependencies between events that are generated by variousdata sources), aggregation dependencies, replication dependencies,control dependencies (for example, dependencies that exist betweenvarious data sources for performing a control operation), actuationdependencies, and availability management (for example, informationpertaining to availability of data to indicates, for example, that datashould be stored in an intermediary data source for ensuring a desiredlevel of data availability). In one implementation, the extended logicallayer 708 is implemented at the base station 104 and at each data sourcein the network 100.

The data management stack 700, in the embodiment shown in FIG. 7, alsocomprises an execution layer 710. The execution layer 710 provides anabstract view and snap-shot, both static and dynamic, of the executionstate of the wireless sensor network 100. The execution layer 710maintains information pertaining to aggregation points in the network100, adaptive query operations, query execution points, and replicationpoints. The information maintained by the execution layer 710 is updatedin response to every occurrence of an event of interest (that is, everyevent that is subscribed to) in the network 100.

The data management stack 700, in the embodiment shown in FIG. 7, alsocomprises a physical layer 712 in which network-specific dependenciesare managed. The topology, routing and base-band issues are managed bythe physical layer 712. The execution layer 710 interacts with and usesthe services provided by the physical layer 712 to publish events,identify aggregation points and optimize data management functionality.

As noted above, an event-based formalism is used for data management inthe wireless sensor network 100. The formalism includes the grammar usedfor specifying the subscriptions and resulting publications of data fromthe sensor network. In one embodiment, the event-based formalism isextended to include a set of operators (also referred to here as“discrete event operators”) that are used for forming an execution planthat can be subjected to analysis and optimization using the propertiesof the operators. The discrete event operators are defined from analgebraic formalism. This algebraic formalism is also referred to hereas a “discrete event process algebra.” The discrete event operators inthis algebra are extended and modified appropriately for execution plancreation.

The discrete event process algebra is used by the data management stack700 of the wireless sensor network 100 to formulate “processes” for usein the execution planning phase performed by the query formalism layer700. The following describes one embodiment of a discrete event processalgebra. In this embodiment, a “trajectory” is defined as the sequenceof events that are accepted and processed by a given process untiltermination of that process. The symbol “ε” is defined as the stringwhose symbols are the events from the event set. The symbol “Σ” isdefined as the set of all the events that are applicable for a process,including both the events that are accepted and rejected by the process.The symbol “Xi” is defined as the string of events that are rejected bya process before an event is successfully accepted by the process. Thesymbol “σi” is defined as the event that is accepted by a process. Thesymbol “Σext” is defined as the event set Σ that is extended with eventsTermination, Divergence, Positive Response and Feedback Response.

In one embodiment, any decision point along the wireless sensor network100 is defined as a process. Formally, a process P in the wirelesssensor network 100 is a subset P⊂O_(td):=(2exp(Σ_(ext))X Σ)*X2exp(Σ_(ext)) satisfying the following conditions:

-   -   Condition 1: (ε, Φ)         P; Null trajectory is in every process. This is the idle state        of the process.    -   Condition 2: ((X₀, σ₁)(X₁, σ₂) . . . (X_(k1)−, σ_(k)), X_(k))         P & ∃j: 0≦j≦k−1; σ_(j+1)         X_(j)         ((X₀, σ₁) . . . (X_(j−1), σ_(j)), X_(j) ∪{        })        P; All trajectories of a non-divergent process must be valid.    -   Condition 3: Termination symbol         is a standalone symbol.    -   Condition 4: A process terminates with the termination event         and accepts no further events.    -   Condition 5: A process generates event (/^) in response a set of        accepted events.

In such an embodiment of a discrete event process algebra, an event setΣ is augmented using /^ and ^\ symbols. In such an embodiment, thefollowing discrete event operators are defined:

(a) Prefix Operator (->.): The prefix operator in the data managementstack 700 is used for sequencing the operations in the wireless sensornetwork 100. The prefix operator Q:=σ->P specifies that the process Qstarts at an initial state and then moves to (transitions to) process Pon event σ. An example of use of prefix operator is the ACTIVATE clauseof the data model (describe above in FIG. 8). One example of asubscription that includes this operator is:

-   -   SUBSCRIBE EVENT (‘124’, ‘125’, ‘126’) OF ‘902’ WHEN (‘512’ OF        ‘345’>‘903’ OF ‘567’) ACTIVATE ‘10:32:12’ LIFETIME ‘12:30:00’        EVERY ‘00:00;10’;

In this example, P and Q are defined as:

-   -   Q=Idle process Δ that is waiting for an event.    -   P=(‘512’ OF ‘345’>‘903’ OF ‘567’)    -   σ=Timer event ‘10:32:12’    -   Δ :=“10:32:12”- - - >(‘512’ OF ‘345’>‘903’ OF ‘567’)

(b) Controlled Alternative Operator (+): This operator enablestransition to two different processes that depend on two different andmutually exclusive events. Given Q1=ρ1->P1 and Q2=σ2->P2 the operator +enables transition from the start state of process Q to either P1 or P2depending upon either the σ1 or σ2 event respectively.Q=(σ1+P1)+(σ2+P2). One example of a subscription that includes thisoperator is:

-   -   SUBSCRIBE EVENT (‘124’) OF ‘902’ WHEN ((‘512’ OF ‘345’>50 AND        ‘602’ OF ‘831’>70) OR (‘602’ OF ‘831’<20 AND ‘129’ OF ‘513’<90))        ACTIVATE IMMEDIATE;

In this example, P1, P2, σ1, σ2, and Q are defined as follows:

-   -   P1=(‘512’ OF ‘345’>50 AND ‘602’ OF ‘831’>70)    -   P2=(‘602’ OF ‘831’<20 AND ‘129’ OF ‘513’<90)    -   σ1=(‘602’ OF ‘831’>70)    -   σ2=(‘602’ OF ‘831’<20)    -   Q=Δ (which is referred to here as the “idle process”) and is        defined as:    -   Δ=((‘602’ OF ‘831’>70)->(‘512’ OF ‘345’>50 AND ‘602’ OF        ‘831’>70)+((‘602’ OF ‘831’<20)->(‘602’ OF ‘831’<20 AND ‘129’ OF        ‘513’<90))

(c) Uncontrolled Alternative Operator (⊕): This operator is not used inthis embodiment of a discrete event process algebra, as the wirelesssensor network 100 in this embodiment is deterministic and nonon-deterministic operators are possible.

(d) Event-Internalization (\σ): This operator is used to remove theoccurrences of a given event from external view. GivenP=(a->b->Δ)+(c->Δ), P\a is given by (b->Δ)+(c->Δ). An example of oneapplication of this operator is during the LIFETIME and EVERY clauses ofthe grammar defined in FIG. 8. The event for LIFETIME clause isinternalized with that of EVERY clause so that LIFETIME events are made‘invisible’ from the external view.

(e) Parallel Composition without Sync. (.∥Φ.): This operator enables twoprocesses to operate in parallel completely independent of each other.This operator is used in the data management stack 700 for performingoperations that need not be synchronized by timer events. One class ofoperation for which this operator is applicable is the ‘Decomposableaggregation’ operation.

(f) Parallel Composition with full Synchronization (.∥Σ.): This operatorenforces a rule that all the events in the event set Σ be completed andfully synchronized. This operator is used in the data management stack700 to execute events that are conjoined by the AND logical operator inthe WHEN clause of the grammar set forth above in FIG. 8. One example ofa query that uses this operator is:

-   -   SUBSCRIBE EVENT (‘124’) OF ‘902’ WHEN ((‘512’ OF ‘345’>50 AND        ‘602’ OF ‘831’>70) OR (‘602’ OF ‘831’<20 AND ‘129’ OF ‘513’<90))        ACTIVATE IMMEDIATE;

In this example, the synchronization set is:

-   -   Σ1={(‘512’ OF ‘345’>50), (‘602’ OF ‘831’>70)}    -   Σ2={(‘602’ OF ‘831’<20), (‘129’ OF ‘513’<90)}

In this example, the event-sets Σ1 and Σ2 are synchronized using the“.∥Σ.” operator.

(g) Parallel Composition on Synchronization Set (.∥A.): This operator issimilar in functionality to the .∥Σ. operator except that A⊂E. Thisrestricted set is applicable in the data management stack 700 when theWHEN clause of a SUBSCRIBE statement contains an OR logical operatorA=Σ(AND) event−Σ(OR) events.

(h) Prioritized Synchronization Composition (.A∥B.): The prioritizedsets are defined on the processes and synchronization is enforced onthese events only. This operator is used in the data management stack700 to enforce priority on the events of a data source. Only thoseevents that have priority above a certain threshold are included in thepriority event sets A and B.

(i) Termination (

): A process terminates if after processing a set of events the processrefuses any further events. A termination operator is used in the datamanagement stack 700 to denote the completion of the SUBSCRIPTION. Avariation of the

 operator is the instance termination operator (↓), which terminates thecurrent instance of SUBSCRIPTION and initializes the subscription for anew execution.

(j) Divergence (

): The divergence operator models the catastrophic behavior of theprocess and it is applicable when the process reaches a chaotic state.In the data management stack 700, this operator is used for modeling thebehavior of the wireless sensor network 100 when either the node dies orthe node gets disconnected from the network.

(k) Positive Response Operator (/^): This operator generates events fromthe process in response to the processing function's output. This is apositive response as it contains values that are generated. An exampleof this kind of operator would be used when values are PUBLISHED fromthe data source in response to an aggregation operation.

(l) Feedback Response Operator (^\): The feedback response operator isused to signal the completion of some task. In the data management stack700, this operator is used to start the operation of parameterassimilation specified in a SUBSCRIPTION/PUBLICATION list. One exampleof a subscription query that uses this operator is:

-   -   SUBSCRIBE EVENT (‘124’) OF ‘902’ WHEN ((‘512’ OF ‘345’>50 AND        ‘602’ OF ‘831’>70) OR (‘602’ OF ‘831’<20 AND ‘129’ OF ‘513’<90))        ACTIVATE IMMEDIATE;

FIG. 9 is a flow diagram of one embodiment of a method 900 of compilinga source form of a query in order to generate the binary form of thatquery. The embodiment of method 900 shown in FIG. 9 is described here asbeing implemented using the wireless sensor network 100, wireless sensornode 102, and data management stack 700 of FIGS. 1, 2, and 7,respectively. Other embodiments are implemented in other ways.

Method 900 includes receiving a query in source form (block 902). Forexample, in one usage scenario, a user of the wireless sensor network100 formulates a query using the query language described above inconnection with FIG. 8 and inputs (or otherwise communicates) the queryto the base station 104. In one implementation, the query formalismlayer 702 implements the functionality that receives the query in sourceform.

The query is checked for syntactic and semantic errors (block 904). Ifthere are any errors, the user is informed of the error (block 906) andprocessing of the received query is terminated. If there are no errors,a graph is generated from the source form of the query (block 908). Thegraph that is generated expresses the query in an instruction set thatis used in the wireless sensor network 100. The source form of the queryis parsed and the corresponding graph is generated based on the contentsof the query. Each node in the graph that is generated is associatedwith a particular instruction from the instruction set. In thisembodiment, the instruction set comprises a set of discrete eventoperators from a discrete event process algebra (also referred to hereas “DEO instructions”) and a set of operators that are used to defineand/or retrieve a value for a simple event (also referred to here as“simple-event instructions”). A discrete event operator instruction isassociated with each parent node in the graph (that is, each node thatincludes one or more child nodes). The discrete event operatorinstruction defines a relationship (for example, sequencing, orderingand/or synchronization) of the subject defined by and under each childnode of that parent node. The DEO instruction associated with eachparent node is also referred to here as the “parent instruction” forthat parent node. Each child node can itself be a parent node (having anassociated DEO instruction and one or more child nodes) or a leaf node.Each leaf node is associated with a single, simple event and asimple-event instruction is associated with each leaf node.

Method 900 further comprises attempting to optimize the query asexpressed in the generated graph (block 910). For example, one or moreof the optimization techniques described above are performed by the DEVlayer 704 of the data management stack 700. A binary form of the queryis then generated from the optimized graph (block 912). The graphgenerated for the query is traversed and the instruction associated witheach node is generated, properly populated, and added to the end of thebinary form of the query. In this way, the binary form of the query isgenerated.

FIG. 10 is a block diagram illustrating one exemplary instruction format1000 for use with the embodiment of method 900 shown in FIG. 9. Eachinstruction includes an operation code (also referred to here as an“opcode”) field 1002 that identifies the particular operation for thatinstruction. Each instruction also includes an operand size field 1004that contains the total size of the operands (if any) included in thatinstruction. In the embodiment shown in FIG. 10, the opcode field 1002and the operand size field 1004 are each 4 bits wide and are located inthe first byte of the instruction (labeled “Byte 0” in FIG. 10). Eachinstruction also includes a parent identifier field 1006 that containsan identifier that identifies the parent instruction for thatinstruction (which corresponds to a particular parent node in acorresponding graph). The parent identifier field 1006, in theembodiment shown in FIG. 10, is one byte wide and is located in thesecond byte of the instruction (labeled “Byte 1” in FIG. 10). Eachinstruction includes none, one, or more operands depending on theparticular opcode specified for that instruction. In the embodimentshown in FIG. 10, each operand follows the parent identifier field 1006in the instruction.

FIG. 11 illustrates one example of an instruction set 1100. Theinstruction set 1100 includes a set of DEO instructions 1102, each ofwhich corresponds to an operator in the discrete event process algebradescribed above. The instruction set 1100 also includes a set ofsimple-event instruction 1104 that are used to define and retrieve asimple event.

FIG. 12 illustrates one example of a query 1200 expressed in source formusing the grammar set forth above in FIG. 8. The query 1200 shown inFIG. 12 specifies a subscription request for getting data from aparticular region of interest. In the query 1200 shown in FIG. 12, theparticular region of interest for the subscription has a regionidentifier of 99. This subscription subscribes to three events thatrelated to the parameters identified by the parameter identifiers of124, 125, and 126. For example, in one implementation, these threeparameters relate to temperature, airflow and light intensity,respectively. In the query shown in FIG. 12, a lifetime of one minuteand thirty seconds is specified for the subscription. The query 1200specifies that the subscription be activated after five seconds and beevaluated every 30 minutes. Additional conditions are specified for thesubscription request in a “WHEN” clause. These additional conditionsspecify when the parameters of interest should be fetched and publishedas events at the specified points of time and when the conditionspecified in the WHEN clause, as a whole, is valid.

FIG. 13 illustrates a graph 1300 generated from the query 1200 that isexpressed in source form in FIG. 12. Each parent node of the graph 1300is displayed in FIG. 13 using the discrete event process algebra symbolthat represents the discrete event operator associated with that parentnode. Also, the discrete event operator associated with each parent nodecan also be viewed as a complex event, each occurrence of which isdetermined as a function of the child nodes of that parent node. Eachleaf node in the graph 1300 is displayed in FIG. 13 using a descriptionof the particular simple event associated with that leaf node. Also,each node of the graph 1300 has an associated index number (referred tohere as an “event index”). The event index for each node in the graph1300 shown in FIG. 13 is set forth in parentheses to the left of thatnode. Each node is also refereed to here using the event index for thatnode (for example, the node having an event index of 1 is referred tohere as “node 1” or “event 1” and the node having an event index of 2 isreferred to here as “node 2” or “event 2”). Each node shown in FIG. 13also has an associated identifier number (referred to here as an “eventidentifier”) that is set forth to the right of or below that node.

FIG. 14 is a table 1400 having a row for each node in the graph 1300shown in FIG. 13. The table 1400 also includes a first column thatidentifies the event index of the node of each row, a second column thatidentifies the event identifier for each row, a third column thatidentifies the event index of the parent event for each row, a fourthcolumn that identifies the event identifier for the parent event foreach row in decimal form, and a fifth column that sets forth the eventidentifier of the parent event for each row in hexadecimal form.

The binary form of the query 1200 set forth in FIG. 12 is shown in FIG.15. The binary form of the query 1200 is shown in FIG. 15 in a grid inwhich the first byte of the binary form of the query 1200 (that is,“00”) is shown in the upper left corner of the grid (that is, at thecell at row 0 and column 0). Each instruction is surrounded by a boxthat is referenced in FIG. 15 with the event index for the node (and thecomplex event represented by that node) to which that instructioncorresponds. For example, the first instruction shown in FIG. 15 is theinstruction that corresponds to the node 1 shown in FIG. 14. The firstinstruction is “00 FF”, where the first 4 bits of the first byte (“0”)is the opcode for a SUBSCRIBE DEO instruction, the second 4 bits of thefirst byte (“0)” is the size of any operands for that opcode (which iszero because there are no operands for that opcode), and the second byteis the event identifier for that instruction's parent event (which is“FF” for the first instruction). The eleventh instruction shown in FIG.15, for example, is the instruction that corresponds to node 11 shown inFIG. 14. The eleventh instruction is “E5 15 2D 0B 63 01 32”, where thefirst 4 bits of the first byte (“E”) is the opcode for the EVENTexternal-event operator shown in FIG. 11, the second 4 bits of the firstbyte (“5”) is the number of operands, in bytes, for this opcode, and thesecond byte (“15”) is the event identifier, in hexadecimal notation, forthat instruction's parent event. The next five bytes (that is, the firstthree operands for this instruction) specify a particular event: whenparameter “45” (“2D” in hexadecimal notation) associated with (opcode“B”) region “99” (“63” in hexadecimal notation) is greater than (opcode“01”) “50” (“32” in hexadecimal notation).

In this example, the binary form of the query 1200 is injected into thewireless sensor network 100. As the query 1200 propagates among thenodes of the wireless sensor network 100, each node that receives binaryform of the query 1200 from a peer node parses the binary form of thequery 1200 and determines if that receiving node is able to act as asource entity for any of the simple events (specified by simple-eventinstructions 1104) specified in the binary form of the query 1200. Ifthe receiving node is able to act as a source entity for such a simpleevent, the receiving node informs the peer node from which the receivingnode received the binary form. As this process plays out throughout thenetwork 100, a series of recursive subscriptions are formed, as managedby the DEV layer 704 of the data management stack 700. FIG. 16illustrates one example of a set of recursive subscriptions that canresult from the query 1200 of FIG. 12. As shown in FIG. 16, a set ofeight subscriptions (labeled S1 through S8 in FIG. 16) are generated inorder to satisfy the query 1200. In this example, the query 1200 isinjected into the wireless sensor network 100 from the base station 104and the base station 104, in this example, is the sink entity for theoriginal subscription generated from the query 1200 (which issubscription S1 in FIG. 16). The source entity for the subscription S1is an intermediary node (labeled node IM1 in FIG. 16). The node IM1publishes events related to parameters 124, 125, and 126 when the WHENclause and the LIFETIME, EVERY, and ACTIVATE clauses of the subscriptionS1 are all true. In this example, the node IM1 is able to act as asource of the time events specified in the LIFETIME, EVERY, and ACTIVATEclauses of the subscription S1. The node IM1 is a sink entity for asubscription S2 for an event related to parameter 124, a sink entity fora subscription S3 for an event related to parameter 125, and a sinkentity for subscription S4 for an event related to parameter 126.

In this example, the source entity for subscription S2 is the node inthe wireless sensor network 100 that is the data source for parameter124 (labeled node 124 in FIG. 16). Node 124 publishes events related toparameter 124 when the WHEN clause and the LIFETIME, EVERY, and ACTIVATEclauses of subscription S2 are all true. In this example, the node 124is able to act as a source of the time events specified in the LIFETIME,EVERY, and ACTIVATE clauses of the subscription S2. The node 124 is asink entity for a subscription S5 for an event related to parameter 45of region 99, a sink entity for a subscription S6 for an event relatedto parameter 54 of region 99, a sink entity for subscription S7 for anevent related to parameter 81 of region 99, and a sink entity for asubscription S8 for an event related to parameter 90 of region 99.

In this example, the source entity for subscription S3 is the node inthe wireless sensor network 100 that is the data source for parameter125 (labeled node 125 in FIG. 16). Node 125 publishes events related toparameter 125 when the WHEN clause and the LIFETIME, EVERY, and ACTIVATEclauses of subscription S3 are all true. In this example, the node 125is able to act as a source of the time events specified in the LIFETIME,EVERY, and ACTIVATE clauses of subscription S3. The node 125 is also asink entity for subscription S5, a sink entity for subscription S6, asink entity for subscription S7, and a sink entity for subscription S8.

In this example, the source entity for subscription S4 is the node inthe wireless sensor network 100 that is the data source for parameter126 (labeled node 126 in FIG. 16). Node 126 publishes events related toparameter 126 when the WHEN clause and the LIFETIME, EVERY, and ACTIVATEclauses of subscription S4 are all true. In this example, the node 126is able to act as a source of the time events specified in the LIFETIME,EVERY, and ACTIVATE clauses of subscription S4. The node 126 is also asink entity for subscription S5, a sink entity for subscription S6, asink entity for subscription S7, and a sink entity for subscription S8.

In this example, the source entity for subscription S5 is the node inthe wireless sensor network 100 that is the data source for parameter 45of region 99 (labeled node 45 in FIG. 16). Node 45 publishes eventsrelated to parameter 45 of region 99 when the WHEN clause and theLIFETIME, EVERY, and ACTIVATE clauses of subscription S5 are all true.In this example, the node 45 is able to act as a source of the timeevents specified in the LIFETIME, EVERY, and ACTIVATE clauses ofsubscription S5 and the data event specified in the WHEN clause ofsubscription S5.

In this example, the source entity for subscription S6 is the node inthe wireless sensor network 100 that is the data source for parameter 54of region 99 (labeled node 54 in FIG. 16). Node 54 publishes eventsrelated to parameter 54 of region 99 when the WHEN clause and theLIFETIME, EVERY, and ACTIVATE clauses of subscription S6 are all true.In this example, the node 54 is able to act as a source of the timeevents specified in the LIFETIME, EVERY, and ACTIVATE clauses ofsubscription S6 and the data event specified in the WHEN clause ofsubscription S6.

In this example, the source entity for subscription S7 is the node inthe wireless sensor network 100 that is the data source for parameter 81of region 99 (labeled node 81 in FIG. 16). Node 81 publishes eventsrelated to parameter 81 of region 99 when the WHEN clause and theLIFETIME, EVERY, and ACTIVATE clauses of subscription S7 are all true.In this example, the node 81 is able to act as a source of the timeevents specified in the LIFETIME, EVERY, and ACTIVATE clauses ofsubscription S7 and the data event specified in the WHEN clause ofsubscription S7.

In this example, the source entity for subscription S8 is the node inthe wireless sensor network 100 that is the data source for parameter 90of region 99 (labeled node 90 in FIG. 16). Node 90 publishes eventsrelated to parameter 90 of region 99 when the WHEN clause and theLIFETIME, EVERY, and ACTIVATE clauses of subscription S8 are all true.In this example, the node 90 is able to act as a source of the timeevents specified in the LIFETIME, EVERY, and ACTIVATE clauses ofsubscription S8 and the data event specified in the WHEN clause ofsubscription S8.

When nodes 45 determines that the LIFETIME, EVERY, ACTIVATE, and WHENclauses of subscription S5 are all true, node 45 (which implements thesource entity for subscription S5) publishes an event related toparameter 45 of region 99 to the sink entities of subscription S5. Whennode 54 determines that the LIFETIME, EVERY, ACTIVATE and WHEN clausesof subscription S6 are all true, node 54 (which implements the sourceentity for subscription S6) publishes an event related to parameter 54of region 99 to the sink entities of subscription S6. When node 81determines that the LIFETIME, EVERY, ACTIVATE and WHEN clauses ofsubscription S7 are all true, node 81 (which implements the sourceentity for subscription S7) publishes an event related to parameter 81of region 99 to the sink entities of subscription S7. When node 90determines that the LIFETIME, EVERY, ACTIVATE and WHEN clauses ofsubscription S8 are all true, node 90 (which implements the sourceentity for subscription S8) publishes an event related to parameter 90of region 99 to the sink entities of subscription S8. The sink entitiesfor subscriptions S5, S6, S7, and S8 are implemented on node 124, node125, and node 126, respectively.

When node 124 determines that the LIFETIME, EVERY, and ACTIVATE clausesof subscription S2 are all true and receives information from node 45and node 54 or from node 81 and node 90 indicating that the WHEN clauseof subscription S2 is true, node 124 (which implements the source entityfor subscription S2) publishes an event related to parameter 124 to thesink entity of subscription S2 (which is implemented on node IM1).

When node 125 determines that the LIFETIME, EVERY, and ACTIVATE clausesof subscription S3 are all true and receives information from node 45and node 54 or from node 81 and node 90 indicating that the WHEN clauseof subscription S3 is true, node 125 (which implements the source entityfor subscription S3) publishes an event related to parameter 125 to thesink entity of subscription S3 (which is implemented on node IM1).

When node 126 determines that the LIFETIME, EVERY, and ACTIVATE clausesof subscription S4 are all true and receives information from node 45and node 54 or from node 81 and node 90 indicating that the WHEN clauseof subscription S4 is true, node 126 (which implements the source entityfor subscription S4) publishes an event related to parameter 126 to thesink entity of subscription S4 (which is implemented on node IM1).

When node IM1 determines that the LIFETIME, EVERY, and ACTIVATE clausesof subscription S1 are all true and receives information from node 124,node 125, or node 126 indicating that the WHEN clause of subscription S1is true (the WHEN clause of subscription S1 is the same as the WHENclause in subscriptions S2, S3, and S4), node IM1 (which implements thesource entity for subscription S1) publishes an event related toparameter 124 (if node IM1 has received a publication for subscriptionS2), parameter 125 (if node IM1 has received a publication forsubscription S2), and/or parameter 126 (if node IM1 has received apublication for subscription S2) to the sink entity of subscription S1(which is implemented on the base station 104).

FIG. 17 is a block diagram of one embodiment of a virtual machine 1700for use in a wireless sensor network. The embodiment of virtual machine1700 shown in FIG. 17 is described here as being implemented using thewireless sensor network 100 and wireless sensor node 102 of FIGS. 1 and2, respectively. Other embodiments are implemented in other ways. Thevirtual machine 1700 shown in FIG. 17 executes one or more “eventprograms” as described below.

In such an embodiment, each event program is the binary form of asubscription that has been installed on the particular node on which thevirtual machine 1700 is implemented (also referred to here as the“target” node) and comprises a set of instructions (for example, fromthe instruction set 1100 shown in FIG. 11). Each event programidentifies one or more simple events (for example, identified using thesimple-event instructions 1104 described above). In the embodiment shownin FIG. 17, the occurrence of each simple event is signaled to thevirtual machine 1700 by an interrupt generated by the underlyinghardware of the target node. As described above, each discrete eventoperator instruction 1102 is also viewed, in the context of the virtualmachine 1700, as a complex event. When each discrete event operatorinstruction 1104 is executed, particular processing is performed by thetarget node. This processing that is performed when each discrete eventoperator instruction 1104 is executed is also referred to here as the“event handler” for the internal event associated with that discreteevent operator instruction 1104.

The embodiment of a virtual machine 1700 shown in FIG. 17 employstechniques used to implement Petri nets. Petri nets are typically usedto model systems with interacting concurrent components where thetransfer of information or materials from one component to anothercomponent requires that the activities of the involved components besynchronized during the interaction. In such a system, it is often thecase that one component must wait for another component in order for thetwo components to remain synchronized. The timing of actions bydifferent components may be very complex and the resulting interactionsbetween components difficult to describe and synchronize. Petri netsprovide a useful mathematical foundation to manage these kinds ofsystems.

A Petri net can be represented as a graph with two types ofnodes—“places” and “transitions.” Each transition has a set of inputplaces and a set of output places. In the graph for a Petri net, eachinput place is shown as being connected to the transition by an arc thatis directed from the input place to the transition. Similarly, eachoutput place is shown as being connected to the transition by an arcthat is directed from the transition to the output place. Places can be“marked” by “tokens.” Each place can hold an integer number of tokens.Transitions whose input places are all marked by at least one token aresaid to be “enabled” and are fired. When a transition is fired, a tokenis placed in each of the output places for that transition.

In the embodiment of virtual machine 1700 shown in FIG. 17, each event(including each simple event and each complex event) in the eventprogram is modeled as a “place.” The event handler for each complexevent (that is, each discrete event operator instruction 1104) ismodeled as a “transition.” Each transition associated with a particularcomplex event has a set of input places comprising the child events ofthat complex event. Each transition associated with a particular complexevent also comprises an output place that corresponds to the parentevent of that complex event. The event handler associated with eachtransition is executed when the transition is enabled. Each transitionis enabled when a logical function associated with that transitionevaluates to “true.” This logical function is defined, in thisembodiment, by a logical operator (for example, AND, OR, or XOR) that isused to logically combine all the input places for the transition, whereeach input place has a logical value of “true” if the input place ismarked and has a logical value of “false” if the input place is notmarked. Each such logical function, in other words, implements the eventfilter condition for the associated complex event.

In such an embodiment, each place (that is, each event) is allocated adata structure that is used to determine if that place has been marked.This data structure is referred to here as a “token data structure” orjust a “token.” A place is marked by storing a value corresponding tological true (for example, a value “1”) in the token for that place. Aplace is not marked when the token for that place contains a valuecorresponding to logical false (for example, a value of “0”). The set oftokens allocated to a set of input places for a given transition (thatis, a given event handler) is also referred to here as the set of “inputtokens” for that transition. Likewise, the token allocated to the outputplace of a given transition is also referred to here as the “outputtoken” for that transition.

The virtual machine 1700 comprises a token management subsystem 1702 forallocating and managing the token data structures used by the virtualmachine 1700. The number of tokens that can be allocated in the virtualmachine 1700 at any time is limited. The token management subsystem 1702includes a token scheduler unit 1704 and a token register bank 1706. Thetoken register bank 1706 comprises a set of bit-addressable tokenregisters 1708 in which the tokens that are allocated by the tokenmanagement subsystem 1702 are stored. In the embodiment shown in FIG.17, the token register bank 1706 includes five token registers 1708,though the token register bank 1706 includes a different number of tokenregisters 1708 in other embodiments. The bits contained within each ofthe token registers 1708 are allocated by the token scheduler unit 1704based on considerations such as runtime management of tokens that havebeen allocated, the allocation of tokens to events such that each tokenresolves to a unique address, and/or the scheduling of tokens for newevents that are generated at run time.

The token management subsystem 1702 also allocates and manages a datastructure (referred to here as the “event-token map” 1710) that mapseach event in the event program to the location within the tokenregister bank 1706 of the token allocated for that event. The tokenmanagement subsystem 1702 also allocates and manages a data structure(referred to here as the “token-association list” 1712) that is used indetermining when a particular event handler should be executed.

In the embodiment shown in FIG. 17, the token-association list 1712 isimplemented as a table in which each event handler in the event programis assigned a row in the table. As noted above, each event handler ismodeled as a transition that has a set of input places and an outputplace. Each row contains a handle (or other reference) to the programinstructions that comprise the event handler associated with that row.Each row also specifies the address (or other identifier) of the inputtokens for the event associated with that row (labeled “Token List” inFIG. 17). Each row also specifies the logical operator (for example,AND, OR, or XOR) that is used to logically combine the input places forthe transition associated with that row's event (labeled “Association”in FIG. 17). The input places are logically combined to determine if thetransition associated with each row is enabled and, as a result, theevent handler associated with that transition should be executed. Thisoperation is also referred to here as “evaluating” that row of thetoken-association list 1712. If the result of logically combining theinput tokens is “true”, then that event handler is executed. When theevent handler is executed for a particular complex event, the eventhandler, among other things, clears (that is, unmarks) the input tokensof that complex event and marks the output token of that complex event.

Every row in the token-association list 1712 is evaluated when any tokenwithin the token register bank 1706 has been marked. The tokenmanagement subsystem 1702 comprises an event handling unit 1714 thatperforms such processing using the token-association list 1712. That is,the event handler 1714, when any token in the token register bank 1706is marked, evaluates each row in the token-association list 1712. If arow evaluates to true, the event handling unit 1714 determines theaddress for the event handler associated with that row using the handle(or other reference) contained in that row and passes the address to theevent-execution management subsystem 1716 (described below) to schedulethe event handler for execution thereby.

The virtual machine 1700 also comprises an interrupt managementsubsystem 1718 that allocates and handles interrupts and timers that areassigned to simple events specified in an event program. The number ofinterrupts and timers that are available on the target node is limited.The interrupt management subsystem 1718 comprises an interruptmultiplexer register bank 1720. The interrupt multiplexer register bank1720 comprises a set of interrupt multiplexer registers 1722. In theembodiment shown in FIG. 17, the interrupt multiplexer register bank1720 includes five interrupt multiplexer registers 1722, though theinterrupt multiplexer register bank 1720 includes a different number ofinterrupt multiplexer registers 1722 in other embodiments.

In the embodiment shown in FIG. 17, the interrupt management subsystem1718 comprises an interrupt multiplexer unit 1726 that allocates thephysical interrupts and timers to the simple events specified in eachevent program. For example, the interrupt multiplexer unit 1726 performssuch processing when each event program is installed on the target node.The interrupt multiplexer unit 1726 identifies all simple events thatare data events (as opposed to timer events) and allocates a physicalinterrupt to each data event. The interrupt multiplexer unit 1726 alsoconfigures the underlying hardware of the target node (for example, theprogrammable processor 206) to generate that physical interrupt onlywhen the condition specified for that data event is true. The interruptmultiplexer unit 1726 also identifies all the simple events that aretimer events (as opposed to data events) and allocates a hardware timerto that timer event. For timer events, the interrupt multiplexer unit1726 also allocates physical interrupt to the timer event and configuresthe underlying hardware of the target node (for example, theprogrammable processor 206) to generate the physical interrupt at theappropriate time as specified in the timer event.

The interrupt multiplexer unit 1726 allocates and maintains a datastructure (referred to here as the “event-interrupt map” 1724) that mapseach simple event specified in the event program to the physicalinterrupt allocated to that event. Each time a physical interruptoccurs, the interrupt multiplexer unit 1726 determines if that physicalinterrupt has been allocated to any event. If the physical interrupt hasbeen allocated to one or more of the events, the interrupt multiplexerunit 1726 “routes” the interrupt to an interrupt handling unit (IHU)1728 included in the interrupt management subsystem 1710. The interrupthandling unit 1728 executes a generic interrupt service routine (ISR)that evaluates each entry in the event-interrupt map 1724. If thephysical interrupt that occurred is contained in a particular entry inthe event-interrupt map 1724, the generic ISR marks the token allocatedto the event specified in that entry of the event-interrupt map 1724.The generic ISR uses the event-token map 1710 to determine which tokenhas been allocated to the event specified in that entry of theevent-interrupt map 1724. As noted above, whenever a token is marked,the event handling unit 1714 evaluates each row in the token-associationlist 1712 to determine if any event handlers should be scheduled forexecution by the event-execution management subsystem 1716.

The virtual machine 1700, in the embodiment shown in FIG. 17, furthercomprises an event-execution management subsystem 1716. Theevent-execution management subsystem 1716 allocates and manages memoryin which the program logic that implements each event handler installedon the target node is stored. The event-execution management subsystem1716 also receives requests from the token management subsystem 1702 toschedule a particular event handler for execution. The event-executionmanagement subsystem 1716 comprises an event scheduler 1730 thatperforms the scheduling of event handlers. The event scheduler 1730schedules event handlers for execution based on factors such aspriority, time to execute and system resources. The event-executionmanagement subsystem 1716 comprises an event-execution unit 1732 thatinteracts with the underlying hardware (for example, the programmableprocessor 206) of the target node to execute the program logic of theevent handlers in accordance with the schedule established by the eventscheduler 1730. The event-execution management subsystem 1716, in theembodiment shown in FIG. 17, comprises an event queue 1734 that is usedby the event-execution scheduler 1730 and the event-execution unit 1732to schedule and execute the program logic of the event handlers,respectively.

In the embodiment shown in FIG. 17, the virtual machine 1700 comprisesan arithmetic and logic unit 1736 that the event-execution managementsubsystem 1716 uses to interact with the underlying programmableprocessor 206 of the target node in order to execute the program logicof the various event handlers installed on the target node. The virtualmachine 1700 further comprises a hardware abstraction unit 1738 that isused to access the other parts of the underlying hardware of the targetnode. For example, in the embodiment shown in FIG. 17, the interruptmanagement subsystem 1710 uses the hardware abstraction unit 1738 tocommunicate with the underlying hardware of the target node about thephysical interrupts and hardware timers used by the virtual machine1700.

In the embodiment shown in FIG. 17, the virtual machine 1700 alsocomprises an event parser 1740 that parses each event program that isinstalled on the target node. During parsing, the event program isstored in event program memory 1742. In one embodiment, the eventprogram memory 1742 is located in a data structure allocated in thememory 208 of the target node. In the embodiment shown in FIG. 17, eachevent program is parsed in two steps. During the first step, the eventparser 1740 identifies each discrete event operator instruction 1102 inthe event program. For each discrete event operator instruction 1102that is identified, the event parser 1740 causes the token schedulerunit 1704 to allocate a token for that complex event and causes theevent-execution management subsystem 1716 to store the program logic forthat event handler. Also, during the first step, the event parser 1740identifies each simple-event instruction 1104 in the event program. Foreach simple-event instruction 1104 that is identified, the event parser1740 causes the token scheduler unit 1704 to allocate a token for thatsimple event and causes the interrupt management subsystem 1718 toallocate a physical interrupt (and a hardware timer, if the simple eventis a timer event). After all of the tokens have been allocated for theevent program, the event parser 1740 performs the second step of theparsing process. In the second step, the event parser orders andsequences the tokens that have been allocated by causing the tokenscheduler unit 1704 to populate a row in the token-association list 1712for each discrete event operator instruction 1102. For each discreteevent operator instruction 1102, a row in the token-association list1712 is populated with the set of input tokens for that discrete eventoperator instruction 1102, the logical operator for combining the set ofinput tokens to determine if the associated event handler should beexecuted, and a handle (or other reference) to the program logic forthat event handler.

In one implementation, the virtual machine 1700 is implemented using the“TinyOS” operating system developed at the University of California atBerkeley. The event scheduler 1730, in such an implementation, isimplemented using the event scheduler component of the TinyOS operatingsystem. In such an implementation, the round-robin scheduling logic ofthe TinyOS scheduler is extended to implement a priority-basedscheduling algorithm for use with the virtual machine 1700 of FIG. 1700.The hardware abstraction unit 1738 of the virtual machine 1700 isimplemented, in such an implementation, using thehardware-abstraction-layer of the TinyOS operating system.

FIG. 18 is a flow chart illustrating the processing of an event programby the embodiment of the virtual machine 1700 shown in FIG. 17. Theprocessing described here as being performed by the virtual machine 1700is carried out by the underlying hardware of the target node (forexample, by the programmable processor 206). The underlying target node,in one implementation, includes a scheduling mechanism that is able to,among other things, allocate processing resources between the processingperformed in method 1800, the program logic for any event handlers thatare executed by the event execution unit 1732, and any processingperformed in order to generate interrupts and management hardwaretimers.

Method 1800 comprises receiving the event program (that is, the binaryform of a subscription) at the virtual machine (block 1802). The eventprogram, in this embodiment, is expressed as a set of instructions,comprising one or more discrete event operator instructions 1102 andsimple-event instructions 1104 of the type shown in FIG. 11. The eventprogram is received by the target node using the underlying networkingand routing layer implemented on the target node and included in thephysical layer 712 of the data management stack 700. The event programis parsed by the event parser 1740 of the virtual machine 1700 (block1804). In this embodiment, the event program is parsed using thetwo-step process described above. In the first step, the tokens for eachevent of the event program (that is, for each instruction) areallocated, the event handler for each discrete event operatorinstruction 1102 is allocated, and the interrupts and hardware timersare allocated for the simple events specified in the event program. Inthe second step, the tokens are ordered and sequenced by populating therows of the token-association list 1712 with the set of input tokens forthe complex event associated with that row, the logical operator forcombining the set of input tokens, and the handle (or other reference)to the program logic for the event handler for that event.

After parsing the event program, the subsystems of the virtual machine1700 are initialized (block 1806). For example, the token registers1708, the interrupt multiplexer registers 1722, and the event scheduler1730 are initialized along with the underlying hardware of the targetnode (for example, the processor 206 that generates interrupts and thehardware timers 222).

After the parsing and initialization is complete, the event program isexecuted by the virtual machine 1700 (block 1808). In the embodimentshown in FIG. 18, the event program is not executed until the ACTIVATEevent specified in the event program has occurred (checked in block1810). In this embodiment, the ACTIVATE event is allocated an interruptand hardware timer by the interrupt management subsystem 1718 and theunderlying hardware of the target node is initialized to generate thisinterrupt when the condition specified in the ACTIVATE instruction ofthe event program is true.

Executing the event program comprises determining when an interrupt hasbeen generated (block 1812). When an interrupt is generated, theinterrupt is checked to determine if that type of interrupt (indicatedby an interrupt identifier) has been allocated to an event in the eventprogram (block 1814). In this embodiment, the interrupt multiplexer unit1726 makes this determination. If that type of interrupt has not beenallocated to an event in the event program, the interrupt is notprocessed (returning back to block 1812). If that type of interrupt hasbeen allocated, the token associated with the event allocated to thatinterrupt is marked (block 1816). In this embodiment, if that type ofinterrupt has been allocated, the interrupt multiplexer unit 1726 routesthe interrupt to the interrupt handler unit 1728. The interrupt handlerunit 1728 executes the generic interrupt service routine, which uses theevent-interrupt map 1724 to determine which event is associated withthat interrupt. The generic interrupt service then marks the tokenallocated to that event, which is stored in the token register bank1706.

After the token is marked, each row in the token-association list 1712is evaluated (block 1818) and if the row evaluates to true (checked inblock 1820), the event handler specified by that row is scheduled forexecution (block 1822). When all rows of the token-association list 1712have been evaluated (checked in block 1824), method 1800 waits for thenext interrupt (returning to block 1812). This is done until theevent-program is terminated, for example, by an event handler (forexample, by an event handler associated with a timer event).

The methods and techniques described here may be implemented in digitalelectronic circuitry, or with a programmable processor (for example, aspecial-purpose processor or a general-purpose processor such as acomputer) firmware, software, or in combinations of them. Apparatusembodying these techniques may include appropriate input and outputdevices, a programmable processor, and a storage medium tangiblyembodying program instructions for execution by the programmableprocessor. A process embodying these techniques may be performed by aprogrammable processor executing a program of instructions to performdesired functions by operating on input data and generating appropriateoutput. The techniques may advantageously be implemented in one or moreprograms that are executable on a programmable system including at leastone programmable processor coupled to receive data and instructionsfrom, and to transmit data and instructions to, a data storage system,at least one input device, and at least one output device. Generally, aprocessor will receive instructions and data from a read-only memoryand/or a random access memory. Storage devices suitable for tangiblyembodying computer program instructions and data include all forms ofnon-volatile memory, including by way of example semiconductor memorydevices, such as EPROM, EEPROM, and flash memory devices; magnetic diskssuch as internal hard disks and removable disks; magneto-optical disks;and DVD disks. Any of the foregoing may be supplemented by, orincorporated in, specially-designed application-specific integratedcircuits (ASICs).

A number of embodiments of the invention defined by the following claimshave been described. Nevertheless, it will be understood that variousmodifications to the described embodiments may be made without departingfrom the spirit and scope of the claimed invention. Accordingly, otherembodiments are within the scope of the following claims.

1. A wireless sensor network comprising: a plurality of nodes thatcommunicate over wireless communication links, wherein at least one ofthe plurality of nodes receives sensor data from a sensor; wherein thewireless sensor network is queried by specifying in a query a set ofevents of interest; wherein for each event of interest included in theset of events of interest, a producer node included in the plurality ofnodes identifies when that event of interest occurs and, when that eventof interest occurs, transmits event data related to that event ofinterest to a consumer node included in the plurality of nodes inresponse to the occurrence of that event of interest; wherein the queryis expressed in a discrete event process algebra that is used to manageprocessing performed to implement the query; and wherein the discreteevent process algebra comprises an operator that defines first andsecond subsets of the set of events of interest, wherein the first andsecond subsets are prioritized, wherein the operator specifies that thefirst and second subsets by synchronized.
 2. The wireless sensor networkof claim 1, wherein the discrete event process algebra comprisesoperators for at least one of sequencing, ordering, and synchronizingthe set of events of interest.
 3. The wireless sensor network of claim1, wherein the discrete event process algebra comprises an operator thatenables two processes to operate in parallel independent of each other.4. The wireless sensor network of claim 1, wherein the discrete eventprocess algebra comprises an operator that specifies that the events ofinterest be completed and synchronized.
 5. The wireless sensor networkof claim 1, wherein the discrete event process algebra comprises anoperator that specifies that a subset of the events of interest becompleted and synchronized.
 6. The wireless sensor network of claim 1,wherein the discrete event process algebra comprises an operator thatterminates at least one event of interest.
 7. The wireless sensornetwork of claim 6, wherein the discrete event process algebra comprisesan operator that terminates a process if after processing the set ofevents the process refuses any further events.
 8. The wireless sensornetwork of claim 6, wherein the discrete event process algebra comprisesan operator that defines behavior of a process for when the processreaches a chaotic state.
 9. The wireless sensor network of claim 8,wherein the process reaches a chaotic state when one of the plurality ofnodes dies or gets disconnected from the wireless sensor network. 10.The wireless sensor network of claim 1, wherein the discrete eventprocess algebra comprises an operator that generates events from aprocess in response to output of an operator.
 11. The wireless sensornetwork of claim 1, wherein the discrete event process algebra comprisesan operator that internalizes at least one of the set of events ofinterest.
 12. The wireless sensor network of claim 1, wherein thediscrete event process algebra comprises an operator that externalizesat least one of the set of events of interest.
 13. The wireless sensornetwork of claim 1, wherein the query is expressed in the discrete eventprocess algebra during compilation of the query.
 14. The wireless sensornetwork of claim 13, wherein the query is compiled to generate a binaryform of the query.
 15. The wireless sensor network of claim 13, whereina tree is generated for the query during the compilation of the query.16. A wireless sensor network comprising: a plurality of nodes thatcommunicate over wireless communication links, wherein at least one ofthe plurality of nodes receives sensor data from a sensor; wherein thewireless sensor network is queried by specifying a set of events ofinterest; and wherein for each event of interest included in the set ofevents of interest, a producer node included in the plurality of nodesidentifies when that event of interest occurs and, when that event ofinterest occurs, transmits event data related to that event of interestto a consumer node included in the plurality of nodes in response to theoccurrence of that event of interest; wherein the wireless sensornetwork further comprises a data management stack that comprises aplurality of layers; wherein the plurality of layers comprises a queryformalism layer that receives a query that identifies the set of eventsof interest and expresses the query in a discrete event process algebrathat is used to manage processing performed to implement the query. 17.The wireless sensor network of claim 16, wherein the plurality of layersof the data management stack further comprises an execution layer thatexecutes on each producer node included in the plurality of nodes. 18.The wireless sensor network of claim 17, wherein the query formalismlayer generates a binary form of the query using the discrete eventprocess algebra.
 19. The wireless sensor network of claim 17, whereinthe binary form of the query is executed by the execution layer of atleast one of the producer nodes.